1. Field of the Invention
The present invention relates generally to a radio frequency (RF) duplexer and a fabricating method thereof. More specifically, the present invention is directed to a compact RF duplexer in which a tuning inductor of a band pass filter at transmitting and receiving ends is formed around a connecting electrode of a printed circuit board (PCB) and a fabricating method thereof.
2. Description of the Related Art
A duplexer is a representative example of elements that use multiple filters. The duplexer properly branches signals transmitted and received over a single antenna in a frequency division duplex (FDD) type communication system so that the same antenna can efficiently both transmit and receive.
The duplexer basically includes a transmitting filter and a receiving filter in addition to an antenna. The transmitting filter is a band pass filter for passing only a frequency for transmission, and the receiving filter is a band pass filter for passing only a frequency for reception. The duplexer enables the transmission and reception over the single antenna by adjusting the frequencies passing through the transmitting and receiving filters.
As mobile communication devices, such as cellular telephones, become increasingly prevalent, demand for compact and lightweight filters used in such devices is also rising for implementation of the duplexer. Film bulk acoustic resonator (FBAR) duplexers are frequently used in the mobile communication devices, since the FBAR duplexer can be freely coupled with RF active elements, is compact and lightweight, can be mass-produced through semiconductor processes, and can be easily coupled with a monolithic microwave integrated circuit (MMIC). Also, the FBAR filters can obtain a high quality factor (Q) value, can be used in a micro-frequency band, and, more particularly, can be implemented in Personal Communication System (PCS) and Digital Cordless System (DCS) bands.
Since the difference between the frequencies of the signals transmitted and received through the transmitting filter and the receiving filter is small, the signals influence each other, giving rise to interference. Accordingly, a phase shifter, which isolates the transmitting filter and the receiving filter from each other to prevent mutual interference, may be incorporated in the duplexer to improve performance. The phase shifter generally includes a capacitor and an inductor, and prevents mutual interference by introducing a phase difference of 90° between the frequencies of the transmitted signal and the received signal.
Referring to FIG. 1, the FBAR duplexer includes transmitter FBAR filter 2, receiver FBAR filter 2 4, and a phase shifter 6. The transmitter FBAR filter 2 filters and transmits a signal input from a transmitting terminal 2a through an antenna 1. The receiver FBAR filter 4 filters and transmits a signal received through the antennal 1 to a receiving terminal 4a. The phase shifter 6 is interposed between the antenna 1 and the receiver filter 4, and shifts phases of the transmitted and received signals to prevent signal interference between the transmitter FBAR filter 2 and the receiver FBAR filter 4.
FIG. 2 illustrates a structure of a duplexer provided by Agilent Technologies Co. Referring to FIG. 2, a transmitter band pass filter chip 20 and a receiver band pass filter chip 30 are spaced at a predetermined interval on a printed circuit board (PCB) 10 by wire bonding. A phase shifter 40 is formed between the transmitter band pass filter chip 20 and the receiver band pass filter chip 30. The two filters 20 and 30 have a size of 3 mm×3 mm to 5 mm×5 mm. Tuning inductors 25-1, 25-2, 35-1, and 35-2 are formed in a width of 20˜30 μm in vicinity of pads of the filters 20 and 30. Accordingly, the total size of the Agilent duplexer reaches about 6 mm×12 mm as the tuning inductors occupy certain areas on the board and are wire bonded. However, the size of the duplexer needs to be reduced in order to realize compactness of the RF duplexer.